Coacervation is a liquid/liquid phase separation occurring in polyelectrolyte mixtures, resulting in the formation of two liquid phases: a polyelectrolyte-rich, so called coacervate phase, and a dilute continuous phase mostly devoid of polyelectrolyte. There are two types of coacervation, namely simple coacervation and complex coacervation, depending on whether the coacervate phase is constituted of a single polymer or of an ionic complex of two oppositely charged polymers. In the classical complex coacervation process used for encapsulation of active ingredients, the phase separation is induced such that the coacervate phase readily coats the drops or particles of the active ingredient. For encapsulation purposes, at least one of the polymers used should also be capable of forming a gel upon cooling. The process is conducted above the gelling temperature, and the temperature is reduced after coating the active ingredient (the core material) with complex coacervate phase, leading to temperature-induced gelation. Optionally, this procedure is followed by a hardening step during which the coacervate shell of the capsule is further cross-linked to provide a mechanically stable barrier shell. The classical coacervation process is described in a variety of publications, including WO 96/32017 (Tastemaker), FR 1,165,805 (The National Cash Register Company), MX 9,704,934 (Tastemaker Corporation), U.S. Pat. No. 6,325,951 (Givaudan), EP 0455598 (Warner-Lambert), EP 2150334 (Firmenich), and WO 2004/022221 (Firmenich).
Capsules with shells or matrices not exclusively made of polymers have also been disclosed. U.S. Pat. No. 4,394,287 discloses for example the use of water-insoluble additives such as perlescent materials, metal flakes, optical brighteners and UV absorbers incorporated into a polymer and coated at the surface of the microcapsule. This forms double layer types of capsules with a limited stability and also present a risk of delamination under certain stress conditions. U.S. Pat. No. 4,115,315 describes the use of perlescent particles embedded in the capsule wall. The purpose is to provide a visual effect only and the document is silent about any other functionality of the capsule. In terms of process, the particles are firstly added to the oil phase (internal phase material). The resultant dispersion is then added to an aqueous encapsulation media, during which the particles are said to flush out into the aqueous coacervate before proceeding to a milling and finally hardening. However with such a process, the migration of particles in the gel phase is limited. Consequently the particles are preferably concentrated at the oil/gel interface and not homogeneously distributed within the capsule wall. On the other hand, publication WO 2004/022220 A1 (Southwest Research Institute) relates generally to the preparation of core/shell microcapsules comprising a membrane that is made of a single polymer material and a structuring agent dispersed therein for protection of core materials with at least one oxygen-sensitive ingredient. The disclosure cites in very generic terms, different possible methods of microencapsulation in particular atomization, coacervation and co-extrusion. Although complex coacervation is cited as an option, only simple coacervation is obviously considered unless the structuring agent is added to the surface of the capsules. According to that teaching, the structuring agent can be added either after emulsion formation and/or by addition to the oil phase. Examples are shown to demonstrate the oxygen barrier properties and to describe the use of clay particles (kaolin) combined with a single polymer (gelatine) (simple coacervation) to obtain a membrane with enhanced barrier properties.
WO 2005/072228 A2 (E Ink) relates to the preparation of capsules, especially capsules intended for use in forming electrophoretic media through a coacervation process to better control the size distribution of core-shell capsules either by emulsifying a water-immiscible phase in a preformed coacervate of the protein (which in essence is a classical coacervation process) or by using a limited coalescence process with colloidal alumina as the surface-active particulate material, i.e., forming a solid-stabilized emulsion as a key step of the process.
WO 2009/147119 (Symrise) relates to a capsule with organic/inorganic hybrid wall. This publication describes synthetic polymer core/shell capsules, wherein the term “hybrid” refers to the presence of Si atoms in the silicon polymers synthesized. It does not disclose capsules containing inorganic (or other) particles included in the wall, and it is not related to coacervation methods.
WO 2010/125094 relates to the encapsulation of liquid or pasty active materials by extrusion through a nozzle, where the encapsulating material is described as a crosslinkable, at least partially condensated organically polymerizable, inorganic-organic hybrid material such as organopolysiloxanes. The term “hybrid” refers to the inorganic nature of Si atoms present in the organosilicon polymers used for a matrix encapsulation process.
WO 2011/124706 (BASF) relates to microcapsules having a core material encapsulated within a microcapsular shell useful for encapsulating flavor, perfume or fragrance, where the shell comprises at least one inorganic/hybrid material. Described therein is a process for preparing microcapsules in which a sol/gel precursor is mixed with a fragrance, perfume or flavor to form the oily core liquid, followed by emulsification of this oil to form an oil-in-water emulsion and subsequently undergoing a sol/gel process, resulting in capsules with a metal oxide or an inorganic/organic hybrid shell. This document discloses shell materials where polymeric or organic materials have been added to the shell or the capsule.
Despite these disclosures, there is a need for improved methods to make core-shell capsules having shells with desired mechanical properties, improved barrier properties for encapsulated material, and other desired functional properties. These methods are now provided by the present invention.